Food Processing TechnologyFood Processing Technology

Prepared by:Prepared by:

Samer mudalalSamer mudalal

1.Introduction1.Introduction

•• Food science is a discipline concerned with Food science is a discipline concerned with all technical aspects of all technical aspects of foodfood, beginning , beginning with with harvestingharvesting or or slaughteringslaughtering, and , and ending with its ending with its cookingcooking and consumption. and consumption. It is considered one of the It is considered one of the agricultural agricultural sciencessciences, and is usually considered distinct , and is usually considered distinct from the field of from the field of nutritionnutrition..

Some of the sub disciplines of food science Some of the sub disciplines of food science include:include:

•• FoodFood safetysafety -- thethe causes,causes, preventionprevention andand communicationcommunicationdealingdealing withwith foodbornefoodborne illnessillness

•• FoodFood microbiologymicrobiology -- thethe positivepositive andand negativenegative interactionsinteractionsbetweenbetweenmicromicro--organismsorganisms andand foodsfoods

•• FoodFood preservationpreservation -- thethe causescauses andand preventionprevention ofof qualityqualitydegradationdegradation

•• FoodFood engineeringengineering -- thethe industrialindustrial processesprocesses usedused totomanufacturemanufacture foodfood

•• ProductProduct developmentdevelopment -- thethe inventioninvention ofof newnew foodfood productsproducts

•• SensorySensory analysisanalysis -- thethe studystudy ofof howhow foodfood isis perceivedperceived byby thetheconsumer'sconsumer's sensessenses

•• Food chemistryFood chemistry -- the molecular composition of food and the the molecular composition of food and the involvement of these molecules in chemical reactions involvement of these molecules in chemical reactions

•• Food packagingFood packaging -- the study of how food is packaged to the study of how food is packaged to preserve the food after it has been processed. preserve the food after it has been processed.

•• Molecular gastronomyMolecular gastronomy -- the scientific investigation of the scientific investigation of processes in cooking, social & artistic processes in cooking, social & artistic gastronomicalgastronomicalphenomena phenomena

•• Food technologyFood technology -- the technological aspectsthe technological aspects

•• Food physicsFood physics -- the physical aspects of foods (such as the physical aspects of foods (such as viscosity, creaminess, texture...) viscosity, creaminess, texture...)

aims of The food industries ( food processing)

today

1. To extend the shelf life to allow time for distribution, sales and home storage.

2. To increase variety in the diet by providing a range of attractive flavours, colours, aromas and textures in food (collectively known as eating quality, sensory characteristics or organoleptic quality); .

3. To provide the nutrients required for health (termed nutritional quality of a food).

4. To generate income for the manufacturing company.

Food processing divided into:Food processing divided into:

• Unit operations combination of procedures to achieve the intended changes to the raw materials.

• Unit operations are grouped together to form a process. The combination and sequence of operations determines the nature of the final product.

Consumer demand for foodsConsumer demand for foods

• fewer synthetic additives,• fewer changes during processing.• ‘healthy’ or ‘natural’ image

• This demands effect on food processing industry to launch food products:

�Free from synthetic additives

�low-fat,

� sugar-free

� low-salt

�Supplemented with vitamins, minerals and probiotic cultures

�Organic products

Motivation and changes of food Motivation and changes of food industryindustry

• improved quality assurance and quality control

• reduces production costs• Reduce wastage• increases production efficiency,• automation

• Heat has important influences on food processing in a number of respects:

• it is the most• convenient way of extending the shelf life of foods by destroying enzymic and

• microbiological activity, or by removing water to inhibit deterioration; it changes the

• nutritional and sensory qualities of foods; and generation of heat is a major processing

• cost.

• Unit operations that take place at ambient temperature and involve minimum heating of foods

• operations that heat foods to extend the shelf life or to alter the eating quality;

• operations that remove heat from foods to extend the shelf life with minimal changes in nutritional qualities and sensory characteristics; the final part, is concerned with operations that are integral to a food process but aresupplementary to the main method of processing.

Food processingFood processing

•• Food processingFood processing is the set of methods and is the set of methods and techniques used to transform raw techniques used to transform raw ingredientsingredientsinto into foodfood or to transform food into other forms or to transform food into other forms for for consumptionconsumption by by humanshumans or animals either in or animals either in the home or by the the home or by the food processing industryfood processing industry. . Food processing typically takes clean, Food processing typically takes clean, harvestedharvestedcrops or crops or slaughteredslaughtered and and butcheredbutchered animal animal products and uses these to produce attractive, products and uses these to produce attractive, marketablemarketable and often longand often long--life food products. life food products. Similar process are used to produce Similar process are used to produce animal feedanimal feed..

Food processing methodsFood processing methods

•• Removal of unwanted outer layers, such as Removal of unwanted outer layers, such as potatopotato peeling or the skinning of peeling or the skinning of peachespeaches. .

•• Chopping or slicing e.g. diced Chopping or slicing e.g. diced carrotscarrots. .

•• MincingMincing and macerating and macerating

•• Liquefaction, such as to produce Liquefaction, such as to produce fruit juicefruit juice

•• FermentationFermentation e.g. in e.g. in beer breweriesbeer breweries

•• EmulsificationEmulsification

•• CookingCooking, such as , such as boilingboiling, , broilingbroiling, , fryingfrying, , steamingsteaming or or grillinggrilling

•• Deep fryingDeep frying•• BakingBaking•• MixingMixing•• Addition of gas such as air entrainment for Addition of gas such as air entrainment for breadbread or or gasificationgasification of of soft drinkssoft drinks

•• ProofingProofing•• Spray dryingSpray drying•• PasteurizationPasteurization•• PackagingPackaging

Chapter 3 .Dehydration

• Dehydration (or drying) is defined as ‘the application of heat under controlled conditions to remove the majority of the water normally present in a food by evaporation’ (or in the case of freeze drying by sublimation).

• The main purpose of dehydration is to extend the shelf life of foods by a reduction in water activity This inhibits microbial growth and enzyme activity, but the processing temperature is usually insufficient to cause their inactivation.

• Drying causes deterioration of both the eating quality and the nutritional value of the food.

• Examples of commercially important dried foods are coffee, milk, raisins, and other fruits, pasta, flours (including bakery mixes), beans, nuts, breakfast cereals, tea and spices.

� There are a large number of factors that control the rate at which foods dry, which can be grouped into the following categories

• those related to the processing conditions

• those related to the nature of the food

• those related to the drier design.

2.1.Drying using heated air

• There are three inter-related factors that control the capacity of air to remove moisture from a food:

1. the amount of water vapor already carried by the air

2. the air temperature

3. the amount of air that passes over the food.

• The amount of water vapour in air is expressed as either absolute humidity2 or relative humidity3 (RH) (in per cent).

• Psychrometry is the study of inter-related properties of air–water vapour systems.

• Heat from drying air is absorbed by food and provides the latent heat needed to evaporate water from the surface. The temperature of the air, measured by a thermometer bulb, is termed the dry-bulb temperature. If the thermometer bulb is surrounded by a wet cloth, heat is removed by evaporation of water from the cloth and the temperature falls. This lower temperature is called the wet-bulb temperature. The difference between the two temperatures is used to find the relative humidity of air on the psychrometric chart

• Adiabatic cooling lines are the parallel straight lines sloping across the chart, which show how absolute humidity decreases as the air temperature increases.

Mechanism of drying

• The third factor that controls the rate of drying, in addition to air temperature and humidity, is the air velocity. When hot air is blown over a wet food, water vapor diffuses through a boundary film of air surrounding the food and is carried away by the moving air

• A water vapour pressure gradient is established from the moist interior of the food to the dry air. This gradient provides the ‘driving force’ for water removal from the food.

• The boundary film acts as a barrier to both heat transfer and water vapor removal during drying. The thickness of the film is determined primarily by the air velocity; if the velocity is low, the boundary film is thicker and this reduces both the heat transfer coefficient and the rate of removal of water vapor. Water vapor leaves the surface of the food and increases the humidity of the surrounding air, to cause a reduction in the water vapour pressure gradient and hence the rate of drying. Therefore the faster the air, the thinner the boundary film and hence the faster the rate of drying.

(a) and (b) Drying curves. The temperature and humidity of the drying air are constant and all heat is supplied to the food surface by convection.

Follow: factors affecting on dryingFollow: factors affecting on drying

• The composition and structure of the food has an influence on the mechanism of moisture removal. For example, the orientation of fibres in vegetables (e.g. celery) and protein strands in meat allow more rapid moisture movement along their length than across the structure.

• The amount of food placed into a drier in relation to its capacity (in a given drier, faster drying is achieved with smaller quantities of food).

2.2 Drying using heated surfaces

• Slurries of food are deposited on a heated steel drum. Heat is conducted from the hot surface, through the food, and moisture is evaporated from the exposed surface. The main resistance to heat transfer is the thermal conductivity of the food

• Additional resistance arises if the partly dried food lifts off the hot surface, forming a barrier layer of air between the food and the drum. Knowledge of the rheological properties of the food is therefore necessary to determine the thickness of the layer and the way in which it is applied to the heated surface

2.3 Types of Driers2.3 Types of Driers

2.3.1 Hot-air driers

�Bin driersBin driers are large, cylindrical or rectangular containers fitted with a mesh base. Hot air passes up through a bed of food at relatively low velocities

�Cabinet driers (tray driers)

These consist of an insulated cabinet fitted with shallow mesh or perforated trays, each of which contains a thin (2–6 cm deep) layer of food. Hot air is blown at 0.5–5ms1 through a system of ducts and baffles to promote uniform air distribution over and/or through each tray.

�Tunnel driers

• Layers of food are dried on trays, which are stacked on trucks programmed to move semi continuously through an insulated tunnel, having one or more types of air flow

• Typically a 20m tunnel contains 12–15 trucks with a total capacity of 5000 kg of food.

�Conveyor driers (belt driers)

• Continuous conveyor driers are up to 20m long and 3m wide. Food is dried on a mesh belt in beds 5–15 cm deep. The air flow is initially directed upwards through the bed of food and then downwards in later stages to prevent dried food from blowing out of the bed.

(a) Conveyor drier and (b) three-stage conveyor drier.

�Fluidized-bed driers

• The main features of a fluidised-bed drier are a distributor to evenly distribute the air at a uniform velocity around the bed of material; a plenum chamber below the distributor to produce an homogenous region of air and prevent localised high velocities; and a disengagement or ‘freeboard’ region above the bed to allow disentrainment of particles thrown up by the air.

• Air from the fluidised bed is usually fed into cyclones to separate out fine particles, which are then added back to the product or agglomerated. Above the distributor, mesh trays contain a bed of particulate foods up to 15 cdeep. Hot air is blown through the bed, causing the food to become suspended and vigorously agitated (fluidised), exposing the maximum surface area of food for drying

2.4 Drying Effect on foods

• All products undergo changes during drying and storage that reduce their quality compared to the fresh material and the aim of improved drying technologies is to minimize these changes while maximizing process efficiency.

• The main changes to dried foods are to the texture and loss of flavor or aroma, but changes in color and nutritional value are also significant in some foods.

2.4.1 Texture

• Changes to the texture of solid foods are an important cause of quality deterioration, The loss of texture in these products is caused by gelatinization of starch, crystallization of cellulose , and localized variations in the moisture content during drying, which set up internal stresses.

• These rupture, crack, compress and permanently distort the relatively rigid cells, to give the food a shrunken shrivelled appearance. On rehydration the product absorbs water more slowly and does not regain the firm texture of the fresh material. There are substantial variations in the degree of shrinkage and rehydration with different foods

• In general, rapid drying and high temperatures cause greater changes to the texture of foods than do moderate rates of drying and lower temperatures. As water is removed during drying, solutes move from the interior of the food to the surface. The mechanism and rate of movement are specific for each solute and depend on the type of food and the drying conditions used

• Evaporation of water causes concentration of solutes at the surface. High air temperatures (particularly with fruits, fish and meats), cause complex chemical and physical changes to solutes at the surface, and the formation of a hard impermeable skin. This is termed case hardening and it reduces the rate of drying to produce a food with a dry surface and a moist interior It is minimised by controlling the drying conditions to prevent excessively high moisture gradients between the interior and the surface of the food.

2.4.2 Flavor and aroma

• Heat not only vaporises water during drying but also causes loss of volatile components from the food and as a result most dried foods have less flavour than the original material. The extent of volatile loss depends on the temperature and moisture content of the food and on the vapour pressure of the volatiles and their solubility in water vapour.

• Volatiles which have a high relative volatility and diffusivity are lost at an early stage in drying. Foods that have a high economic value due to their characteristic flavours (for example herbs and spices) are dried at low temperatures

• Flavour changes, due to oxidative or hydrolytic enzymes are prevented in fruits by the use of sulphur dioxide, ascorbic acid or citric acid, by pasteurisation of milk or fruit juices and by blanching of vegetables. Other methods which are used to retain flavours in dried foods include:

� recovery of volatiles and their return to the product during drying

�mixing recovered volatiles with flavour fixing compounds, which are then granulated and added back to the dried product (for example dried meat powders)

� addition of enzymes, or activation of naturally occurring enzymes, to produce flavours from flavour precursors in the food (for example onion and garlic are dried under conditions that protect the enzymes that release characteristic flavours).

2.4.3 Colour

• There are a number of causes of colour loss or change in dried foods; drying changes the surface characteristics of a food and hence alters its reflectivity and colour. In fruits and vegetables, chemical changes to carotenoid and chlorophyll pigments are caused by heat and oxidation during drying and residual polyphenoloxidase enzyme activity causes browning during storage.

• This is prevented by blanching or treatment of fruits with ascorbic acid or sulphur dioxide. For moderately sulphured fruits and vegetables the rate of darkening during storage is inversely proportional to the residual sulphur dioxide content. However, sulphur dioxide bleaches anthocyanins, and residual sulphur dioxide is also linked to health concerns. Its use in dried products is now restricted in many countries.

• The rate of Maillard browning in stored milk and fruit products depends on the water activity of the food and the temperature of storage. The rate of darkening increases markedly at high drying temperatures, when the moisture content of the product exceeds 4–5%, and at storage temperatures above 38ºC

2.4.4 Nutritional value

• Large differences in reported data on the nutritional value of dried foods are due to wide variations in the preparation procedures, the drying temperature and time, and the storage conditions. In fruits and vegetables, losses during preparation usually exceed those caused by the drying operation

• For example Escher and Neukom (1970) showed that losses of vitamin C during preparation of apple flakes were 8% during slicing, 62% from blanching, 10% from pureeing and 5% from drum drying

• Vitamins have different solubilities in water and as drying proceeds, some (for example riboflavin) become supersaturated and precipitate from solution, so losses are small . Others, for example ascorbic acid, are soluble until the moisture content of the food falls to very low levels and these react with solutes at higher rates as drying proceeds. Vitamin C is also sensitive to heat and oxidation and short drying times, low temperatures, low moisture and oxygen levels during storage are therefore necessary to avoid large losses.

Vitamin losses in selected dried foods

2.4.5 Rehydration

• Water that is removed from a food during dehydration cannot be replaced in the same way when the food is rehydrated (that is, rehydration is not the reverse of drying); loss of cellular osmotic pressure, changes in cell membrane permeability, solute migration, crystallisation of polysaccharides and coagulation of cellular proteins all contribute to texture changes and volatile losses and are each irreversible

Chapter 4. Blanching

• Blanching serves a variety of functions, one of the main ones being to destroy enzymatic activity in vegetables and some fruits, prior to further processing by heat. As such, it is not intended as a sole method of preservation but as a pre-treatment which is normally carried out between the preparation of the raw material and later operations (particularly heat sterilisation, dehydration and freezing. Blanching is also combined with peeling and/or cleaning of food, to achieve savings in energy consumption, space and equipment costs

• A few processed vegetables, for example onions and green peppers, do not require blanching to prevent enzyme activity during storage, but the majority suffer considerable loss in quality if blanching is omitted or if they are under-blanched. To achieve adequate enzyme inactivation, food is heated rapidly to a pre-set temperature, held for a pre-set time and then cooled rapidly to near ambient temperatures.

The factors which influence blanching time are:

• type of fruit or vegetable

• size of the pieces of food

• blanching temperature

• method of heating.

3.1 Theory

• The maximum processing temperature in freezing and dehydration is insufficient to inactivate enzymes. If the food is not blanched, undesirable changes in sensory characteristics and nutritional properties take place during storage. In canning, the time taken to reach sterilizing temperatures, particularly in large cans, may be sufficient to allow enzyme activity to take place. It is therefore necessary to blanch foods prior to these preservation operations.

• Under-blanching may cause more damage to food than the absence of blanching does, because heat, which is sufficient to disrupt tissues and release enzymes, but not inactivate them, causes accelerated damage by mixing the enzymes and substrates. In addition, only some enzymes may be destroyed which causes increased activity of others and accelerated deterioration.

• Enzymes which cause a loss of eating and nutritional qualities in vegetables and fruits include lipoxygenase, polyphenoloxidase, polygalacturonase and chlorophyllase. Two heat-resistant enzymes which are found in most vegetables are catalase and peroxidase.

`̀

• Although they do not cause deterioration during storage, they are used as marker enzymes to determine the success of blanching. Peroxidase is the more heat resistant of the two, so the absence of residual peroxidase activity would indicate that other less heat-resistant enzymes are also destroyed.

The factors that control the rate of heating at the centre of the product can be summarized as:

• the temperature of the heating medium

• the convective heat transfer coefficient

• the size and shape of the pieces of food

• the thermal conductivity of the food.

• Blanching reduces the numbers of contaminating micro-organisms on the surface of foods and hence assists in subsequent preservation operations. This is particularly important in heat sterilization, as the time and temperature of processing are designed to achieve a specified reduction in cell numbers.

• Blanching also softens vegetable tissues to facilitate filling into containers and removes air from intercellular spaces which increases the density of food and assists in the formation of a head-space vacuum in cans

3.2 Equipment

• The two most widespread commercial methods of blanching involve passing food through an atmosphere of saturated steam or a bath of hot water. Both types of equipment are relatively simple and inexpensive. Microwave blanching is not yet used commercially on a large scale.

3.2.1 Steam blanchers`

• In general this is the preferred method for foods with a large area of cut surfaces as leaching losses are much smaller than those found using hot-water blanchers.

• At its simplest a steam blancher consists of a mesh conveyor belt that carries food through a steam atmosphere in a tunnel. The residence time of the food is controlled by the speed of the conveyor and the length of the tunnel. Typically a tunnel is 15m long and 1–1.5m wide.

• In conventional steam blanching, there is often poor uniformity of heating in the multiple layers of food. The time–temperature combination required to ensure enzyme inactivation at the centre of the bed results in overheating of food at the edges and a consequent loss of texture and other sensory characteristics.

Advantages and limitations of conventional steam and hot-water blanchers

3.2.2 Hot-water blanchers

• There are a number of different designs of blancher, each of which holds the food in hot water at 70–100ºC for a specified time and then removes it to a dewatering-cooling section.

• In the widely used reel blancher, food enters a slowly rotating cylindrical mesh drum which is partly submerged in hot water. The food is moved through the drum by internal flights. The speed of rotation and length control the heating time. Pipe blanchers consist of a continuous insulated metal pipe fitted with feed and discharge ports. Hot water is recirculated through the pipe and food is metered in. The residence time of food in the blancher is determined by the length of the pipe and the velocity of the water

Blanchers: (a) IQB steam blancher (after Timbers et al. (1984)); (b) blancher–cooler(from Hallstrom et al. (1988)) and (c) counter-current blancher (after Wendt et al. (1983)).

•• IQB: IQB: Individual Quick Blanching

3.3 Blanching Effect on foods

• The heat received by a food during blanching inevitably causes some changes to sensory and nutritional qualities. However, the heat treatment is less severe than for example in heat sterilization, and the resulting changes in food quality are less pronounced. In general, the time–temperature combination used for blanching is a compromise which ensures adequate enzyme inactivation but prevents excessive softening and loss of flavor in the food

3.3.1 Nutrients

• Some minerals, water-soluble vitamins and other water-soluble components are lost during blanching. Losses of vitamins are mostly due to leaching, thermal destruction and, to a lesser extent, oxidation. The extent of vitamin loss depends on a number of factors including:

� the maturity of the food and variety

�methods used in preparation of the food, particularly the extent of cutting, slicing or dicing

�the surface-area-to-volume ratio of the pieces of food

�method of blanching

�time and temperature of blanching (lower vitamin losses at higher temperatures for shorter times)

�the method of cooling

�the ratio of water to food (in both water blanching and cooling).

Effect of blanching on cell tissues: S, starch gelatinised; CM, cytoplasmic membranes altered; CW, cell walls little altered; P, pectins modified; N, nucleus and cytoplasmic proteins denatured; C, chloroplasts and chromoplasts distorted.

Effect of blanching method on ascorbic acid losses in selected vegetables

Differences in both steam versus water blanching and air versus water cooling are significant at the 5% level Adapted from Cumming et al.

• Losses of ascorbic acid are used as an indicator of food quality, and therefore the severity of blanching

3.3.2 3.3.2 Colour and flavour

• Blanching brightens the colour of some foods by removing air and dust on the surface and thus altering the wavelength of reflected light. The time and temperature of blanching also influence the change in food pigments according to their D value. Sodium carbonate (0.125% w/w) or calcium oxide are often added to blancher water to protect chlorophyll and to retain the colour of green vegetables, although the increase in pH may increase losses of ascorbic acid.

• Enzymatic browning of cut apples and potatoes is prevented by holding the food in dilute (2% w/w) brine prior to blanching. When correctly blanched, most foods have no significant changes to flavor or aroma, but under-blanching can lead to the development of off-flavors during storage of dried or frozen foods.

3.3.3 3.3.3 Texture

• One of the purposes of blanching is to soften the texture of vegetables to facilitate filling into containers prior to canning. However, when used for freezing or drying, the time -temperature conditions needed to achieve enzyme inactivation cause an excessive loss of texture in some types of food (for example certain varieties of potato) and in large pieces of food. Calcium chloride (1–2%) is therefore added to blancher water to form insoluble calcium pectate complexes and thus to maintain firmness in the tissues

Chapter 5.Pasteurisation

• Pasteurization is a relatively mild heat treatment, in which food is heated to below 100ºC. In low acid foods (pH>4.5, for example milk) it is used to minimize possible health hazards from pathogenic micro-organisms and to extend the shelf life of foods for several days.

• In acidic foods (pH <4.5, for example bottled fruit) it is used to extend the shelf life for several months by destruction of spoilage micro-organisms (yeasts or moulds) and/or enzyme inactivation

4.1 Theory

• The sensible heat required to raise the temperature of a liquid during pasteurization is found using:

Purpose of pasteurization for different foods

4.2 Equipment

4.2.1 Pasteurization of packaged foods

• Some liquid foods (for example beers and fruit juices) are pasteurized after filling into containers. Hot water is normally used if the food is packaged in glass, to reduce the risk of thermal shock to the container (fracture caused by rapid changes in temperature). Maximum temperature differences between the container and water are 20ºC for heating and 10ºC for cooling.

• Metal or plastic containers are processed using steam–air mixtures or hot water as there is little risk of thermal shock. In all cases the food is cooled to approximately 40ºC to evaporate surface water and therefore to minimize external corrosion to the container or cap, and to accelerate setting of label adhesives.

• Hot-water pasteurisers may be batch or continuous in operation. The simplest batch equipment consists of a water bath in which crates of packaged food are heated to a pre-set temperature and held for the required length of time. Cold water is then pumped in to cool the product. A continuous version consists of a long narrow trough fitted with a conveyor belt to carry containers through heating and cooling stages.

Time–temperature relationships for pasteurization. The hatched area shows the range of times and temperatures used in commercial milk pasteurization.

4.2.2 Pasteurization of unpackaged liquids

• Swept surface heat exchangers (Barclay et al., 1984) or open boiling pans are used for small-scale batch pasteurization of some liquid foods. However, the large scale pasteurization of low viscosity liquids (for example milk, milk products, fruit juices, liquid egg, beers and wines) usually employs plate heat exchangers. Some products (for example fruit juices, wines) also require de-aeration to prevent oxidative changes during storage. They are sprayed into a vacuum chamber and dissolved air is removed by a vacuum pump, prior to pasteurization.

• The plate heat exchanger consists of a series of thin vertical stainless steel plates, held tightly together in a metal frame. The plates form parallel channels, and liquid food and heating medium (hot water or steam) are pumped through alternate channels, usually in a counter-current flow pattern. Each plate is fitted with a synthetic rubber gasket to produce a watertight seal and to prevent mixing of the product and the heating and cooling media. The plates are corrugated to induce turbulence in the liquids and this, together with the high velocity induced by pumping, reduces the thickness of boundary films to give high heat transfer coefficients (3000 – 11 500Wm2K1).

Plate heat exchanger

Counter-current flow through plate heat exchanger: (a) one pass with four channels per medium; (b) two passes with two channels per pass and per medium

Pasteurizing using a plate heat exchanger.

4.3 Effect on foods

• Pasteurization is a relatively mild heat treatment and even when combined with other unit operations (for example irradiation and chilling there are only minor changes to the nutritional and sensory characteristics of most foods. However, the shelf life of pasteurized foods is usually only extended by a few days or weeks compared with many months with the more severe heat sterilization. Minimizing post processing contamination is essential to ensure an adequate shelf life.

4.3.1 Color, flavor and aroma

• In fruit juices the main cause of color deterioration is enzymatic browning by polyphenoloxidase. This is promoted by the presence of oxygen, and fruit juices are therefore routinely deaerated prior to pasteurization. The difference between the whiteness of raw milk and that of pasteurized milk is due to homogenization, and pasteurization alone has no measurable effect.

• A small loss of volatile aroma compounds during pasteurization of juices causes a reduction in quality and may also unmask other ‘cooked’ flavors.

4.3.2 Vitamin loss

• In fruit juices, losses of vitamin C and carotene are minimised by deaeration. Changes to milk are confined to a 5% loss of serum proteins and small changes to the vitamin content

Vitamin losses during pasteurization of milk

Chapter 6. Heat processing using hot oils (Frying)

• Frying is a unit operation which is mainly used to alter the eating quality of a food. A secondary consideration is the preservative effect that results from thermal destruction of micro-organisms and enzymes, and a reduction in water activity at the surface of the food (or throughout the food, if it is fried in thin slices).

• The shelf life of fried foods is mostly determined by the moisture content after frying: foods that retain a moist interior (for example doughnuts, fish and poultry products which may also be breaded or battered), have a relatively short shelf life, owing to moisture and oil migration during storage.

5.1 Theory

• When food is placed in hot oil, the surface temperature rises rapidly and water is vaporised as steam. The surface then begins to dry out in a similar way to that described during baking and roasting. The plane of evaporation moves inside the food, and a crust is formed. The surface temperature of the food then rises to that of the hot oil, and the internal temperature rises more slowly towards 100ºC. The rate of heat transfer is controlled by the temperature difference between the oil and the food and by the surface heat transfer coefficient. The rate of heat penetration into the food is controlled by the thermal conductivity of the food

• The surface crust has a porous structure, consisting of different-sized capillaries. During frying, both water and water vapour are removed from the larger capillaries first, and replaced by hot oil. Moisture moves from the surface of the food through a boundary film of oil, the thickness of which controls the rate of heat and mass transfer.

• The thickness of the boundary layer is determined by the viscosity and velocity of the oil. The water vapour pressure gradient between the moist interior of the food and the dry oil is the driving force behind moisture loss, in a similar way to hot air dehydration

The time taken for food to be completely fried depends on:

•. the type of food

• the temperature of the oil

• the method of frying (shallow or deep-fat frying)

• the thickness of the food

• the required change in eating quality

• The temperature used for frying is determined mostly by economic considerations and the requirements of the product. At high temperatures (180–200ºC), processing times are reduced and production rates are therefore increased. However, high temperatures also cause accelerated deterioration of the oil and formation of free fatty acids, which alter the viscosity, flavour and colour of the oil and promote foaming.

• This increases the frequency with which oil must be changed and hence increases costs. A second economic loss arises from the vigorous boiling of the food at high temperatures which causes loss of oil by aerosol formation and entrainment in the product.

• There are two main methods of commercial frying which are distinguished by the method of heat transfer involved: these are shallow frying and deep-fat frying.

5.2 Shallow (or contact) frying

• This method is most suited to foods which have a large surface-area-to-volume ratio (for example bacon slices, eggs, burgers and other types of patties). Heat is transferred to the food mostly by conduction from the hot surface of the pan through a thin layer of oil

• The thickness of the layer of oil varies as a result of irregularities in the surface of the food. This, together with the action of bubbles of steam which lift the food off the hot surface, causes temperature variations as frying proceeds and produces the characteristic irregular browning of shallow fried foods

Heat and mass transfer in (a) shallow frying and (b) deep fat frying.

5.3 Deep-fat frying

• Here heat transfer is a combination of convection within the hot oil and conduction to the interior of the food. All surfaces of the food receive a similar heat treatment, to produce a uniform color and appearance. Deep-fat frying is suitable for foods of all shapes, but irregularly shaped food or pieces with a greater surface: mass ratio tend to absorb and entrain a greater volume of oil when it is removed from the fryer

5.4 Equipment

• Shallow-frying equipment consists of a heated metal surface, covered in a thin layer of oil. Commercially, continuous deep-fat fryers are more important. In batch operation the food is suspended in a bath of hot oil and retained for the required degree of frying, often assessed by changes in surface color.

• Continuous deep-fat friers consist of a stainless steel mesh conveyor which is submerged in a thermostatically controlled oil tank. They are heated by electricity, gas, fuel oil or steam. Food is metered into the oil by slow-moving paddles and either sinks to a submerged conveyor or, if the food floats, is held below the surface by a second conveyor

Continuous deep-fat frier.

Different conveyor arrangements: (a) delicate non-buoyant products (for example fish sticks); (b) breadcrumb-coated products; (c) dry buoyant bulk products (for example half-product snacks); (d) dual purpose (for example nuts and snacks).

5.5 Effect on foods

• Frying is an unusual unit operation in that the product of one food process (cooking oil) is used as the heat transfer medium in another. The effect of frying on foods therefore involves both the effect on the oil, which in turn influences the quality of the food, and the direct effect of heat on the fried product.

5.5.1 Effect of heat on oil

• Prolonged heating of oils at the high temperatures used in frying, in the presence of moisture and oxygen released from foods, causes oxidation of the oil to form a range of volatile carbonyls, hydroxy acids, keto acids and epoxy acids. These cause unpleasant flavours and darkening of the oil.

• The various breakdown products are classified as volatile decomposition products (VDP) and non-volatile decomposition products (NVDP).

• VDPs have a lower molecular weight than the oil and are lost in vapour from the frier. Analysis of the vapour has indicated up to 220 different components (Nielsen, 1993), which form the smoke and odour of frying. However, these components are also present in the oil and contribute to the flavour of the fried product.

• NVDPs are formed by oxidation and polymerisation of the oil and form sediments on the sides and at the base of the frier. Polymerisation in the absence of oxygen produces cyclic compounds and high-molecular-weight polymers, which increase the viscosity of the oil. This lowers the surface heat transfer coefficient during frying and increases the amount of oil entrained by the food.

• Oxidation of fat-soluble vitamins in the oil results in a loss of nutritional value. Retinol, carotenoids and tocopherols are each destroyed and contribute to the changes in flavour and colour of the oil.

• the preferential oxidation of tocopherols has a protective (antioxidant) effect on the oil. This is particularly important as most frying oils are of vegetable origin and contain a large proportion of unsaturated fats which are readily oxidised.

5.5.2 Effect of heat on fried foods

• The main purpose of frying is the development of characteristic colours, flavours and aromas in the crust of fried foods. These eating qualities are developed by a combination of Maillard reactions and compounds absorbed from the oil. The main factors that control the changes to colour and flavour in a given food are therefore:

� the type of oil used for frying

� the age and thermal history of the oil

� the interfacial tension between the oil and the product

� the temperature and time of frying

� the size, moisture content and surface characteristics of the food

�post-frying treatments

• Each of these factors, together with any pre-treatments, such as blanching or partial drying, also influences the amount of oil entrained within the food.

• Where fried foods form a large part of the diet, excess fat consumption can be an important source of ill-health, and is a key contributor to obesity, coronary heart disease and perhaps some types of cancer (Browner et al., 1991). These risks and consumer trends towards lower fat products is creating pressure on processors to alter processing conditions to reduce the amount of oil absorbed or entrained in their products.

• The texture of fried foods is produced by changes to proteins, fats and polymeric carbohydrates which are similar to those produced by baking Changes to protein quality occur as a result of Maillard reactions with amino acids in the crust.

• The fat content of the food increases owing to oil absorption and entrainment, but the nutritional significance of this is difficult to determine as it varies according to a number of factors including the type and thermal history of the oil, and the amount entrained in the food.

• The effect of frying on the nutritional value of foods depends on the type of process used. High oil temperatures produce rapid crust formation and seal the food surface. This reduces the extent of changes to the bulk of the food, and therefore retains a high proportion of the nutrients.

• For example a 17% loss of available lysine is reported in fried fish, although this increased to 25% when thermally damaged oil was used (Tooley, 1972). Shallow-fried liver lost 15% thiamin

Chapter 7. Irradiation

• Ionising radiation takes the form of -rays from isotopes or, commercially to a lesser extent, from X-rays and electrons. It is permitted in 38 countries to preserve foods by destruction of micro-organisms or inhibition of biochemical changes.

The main advantages of irradiation are as follows:

� there is little or no heating of the food and therefore negligible change to sensory characteristics

�packaged and frozen foods may be treated

� fresh foods may be preserved in a single operation, and without the use of chemical preservatives

� energy requirements are very low

� changes in nutritional value of foods are comparable with other methods of food preservation

� processing is automatically controlled and has low operating costs.

major disadvantages

� the process could be used to eliminate high bacterial loads to make otherwise unacceptable foods saleable

� if spoilage micro-organisms are destroyed but pathogenic bacteria are not, consumers will have no indication of the unwholesomeness of a food

� there will be a health hazard if toxin-producing bacteria are destroyed after they have contaminated the food with toxins

� the possible development of resistance to radiation in micro-organisms

� loss of nutritional value

�until recently, inadequate analytical procedures for detecting whether foods have been irradiated

� public resistance due to fears of induced radioactivity or other reasons connected to concerns over the nuclear industry.

• These concerns have been addressed by the Joint FAO/IAEA/WHO Expert Committee on the Wholesomeness of Irradiated Food (JECFI) who concluded that the maximum average dose of 10 kGy ‘presents no toxicological hazard and no special nutritional or microbiological problems in foods’

Applications of food irradiation

6.1 Theory

• -rays and electrons are distinguished from other forms of radiation by their ionising ability (that is they are able to break chemical bonds when absorbed by materials). The products of ionisation may be electrically charged (ions) or neutral (free radicals). These then further react to cause changes in an irradiated material known as radiolysis. It is these reactions that cause the destruction of micro-organisms, insects and parasites during food irradiation.

• In foods that have a high moisture content, water is ionised by radiation. Electrons are expelled from water molecules and break the chemical bonds. The products then recombine to form hydrogen, hydrogen peroxide, hydrogen radicals (H), hydroxyl radicals (OH) and hydroperoxyl radicals (HO2)

• The radicals are extremely short lived (less than 10-5 s) but are sufficient to destroy bacterial cells.

(a) Ionisation of water (after Robinson (1986)); (b) formation of free radicals during irradiation (after Hughes (1982)).

6.3 Equipment

Isotope irradiation plant: 1, irradiation chamber; 2, control room; 3, infeed conveyor; 4, outlet conveyor; 5, raw food store; 6, irradiated product store; 7, concrete shielding wall

6.4 Effect on micro-organisms

• The reactive ions produced by irradiating foods injure or destroy micro-organisms immediately, by changing the structure of cell membranes and affecting metabolic enzyme activity. However, a more important effect is on deoxyribonucleic acid (DNA) and ribonucleic acid molecules in cell nuclei, which are required for growth and replication

Microbial destruction by irradiation: A, Pseudomonas sp.; B, Salmonella sp.; C, Bacillus cereus; D, Deinococcus radiodurans; E, typical virus.

D-values of important pathogens

6.5 6.5 Nutritional and sensory value

Effect of irradiation on water-soluble vitamins in selected foods

A comparison of vitamin contents of heat sterilised and irradiated (58 kGy at 25ºC) chicken meat

6.6 Effect on packaging

Chapter 8. Chilling

• Chilling is the unit operation in which the temperature of a food is reduced to between 1ºC and 8ºC. It is used to reduce the rate of biochemical and microbiological changes, and hence to extend the shelf life of fresh and processed foods. It causes minimal changes to sensory characteristics and nutritional properties of foods and, as a result, chilled foods are perceived by consumers as being convenient, easy to prepare, high quality and ‘healthy’, ‘natural’ and ‘fresh’.

• Chilling is often used in combination with other unit operations (for example fermentation or pasteurization) to extend the shelf life of mildly processed foods. There is a greater preservative effect when chilling is combined with control of the composition of the storage atmosphere than that found using either unit operation alone.

Chilled foods are grouped into three categories according to their storage temperature range as follows:

1. -1ºC to +1ºC (fresh fish, meats, sausages and ground meats, smoked meats and breaded fish).

2. 0ºC to +5ºC (pasteurised canned meat, milk, cream, yoghurt, prepared salads, sandwiches, baked goods, fresh pasta, fresh soups and sauces, pizzas, pastries and unbaked dough).

3. 0ºC to +8ºC (fully cooked meats and fish pies, cooked or uncooked cured meats, butter, margarine, hard cheese, cooked rice, fruit juices and soft fruits).

7.1 Theory

7.1.1 Fresh foods

• The rate of biochemical changes caused by either micro-organisms or naturally occurring enzymes increases logarithmically with temperature. Chilling therefore reduces the rate of enzymic and microbiological change and retards respiration of fresh foods

The factors that control the shelf life of fresh crops in chill storage include:

� • the type of food and variety or cultivar

� • the part of the crop selected (the fastest growing parts have the highest metabolic rates and the shortest storage lives )

� • the condition of the food at harvest (for example the presence of mechanical damage or microbial contamination, and the degree of maturity)

� • the temperature of harvest, storage, distribution and retail display

� • the relative humidity of the storage atmosphere, which influences dehydration losses.

Botanical function related to respiration rate and storage life for selected products

• To chill fresh foods it is necessary to remove both sensible heat (also known as field heat) and heat generated by respiratory activity. The production of respiratory heat at 20ºC and atmospheric pressure is given by equation

Optimum storage conditions for some fruits and vegetables

7.1.2 Processed foods

• There are four broad categories of micro-organism, based on the temperature range for growth

1. thermophilic (minimum: 30–40ºC, optimum: 55–65ºC)

2. mesophilic (minimum: 5–10ºC, optimum: 30–40ºC)

3. psychrotrophic (minimum: 0–5ºC, optimum: 20–30ºC)

4. psychrophilic (minimum: 0–5ºC, optimum: 12–18ºC).

• Chilling prevents the growth of thermophilic and many mesophilic micro-organisms. The main microbiological concerns with chilled foods are a number of pathogens that can grow during extended refrigerated storage below 5ºC, or as a result of any increase in temperature (temperature abuse) and thus cause food poisoning

• Examples of these pathogens that survive chilling conditions are Aeromonas hydrophilia, Listeria spp, Yersinia enterocolitica, some strains of Bacillus cereus, Vibrio parahaemolyticus and enteropathogenic Escherichia coli (Marth, 1998). An example of the last (E.coli 0157:H7) may cause hemorrhagic colitis after ingestion of as little as ten cell

Pathogenic or spoilage bacteria in high–risk chilled foods

The shelf life of chilled processed foods is determined by:

• the type of food

• the degree of microbial destruction or enzyme inactivation achieved by the process

• control of hygiene during processing and packaging

• the barrier properties of the package

• temperatures during processing, distribution and storage.

7.1.3 Cook–chill systems

• Individual foods (for example sliced roast meats) or complete meals are produced by cook–chill or cook–pasteurise–chill processes

• These products, which include complete meals or components such as sauces, were developed for institutional catering to replace warm-holding,1 which reduces losses in nutritional and eating quality and is less expensive

The range of chilled foods can be characterised by the class of microbial risk that they pose to consumers as follows:

�Class 1 foods containing raw or uncooked ingredients, such as salad or cheese as ready-to-eat (RTE) foods (also includes chill-stable raw foods, such as meat, fish, etc.)

�Class 2 products made from a mixture of cooked and low risk raw ingredients

�Class 3 cooked products that are then packaged�Class 4 products that are cooked after packaging,

including ready-to-eat-products for- extended-durability (REPFEDs) having a shelf life of 40+ days (the acronym is also used to mean refrigerated-pasteurised-foods-for-extended durability).

• After preparation, cooked–chilled foods are portioned and chilled within 30 min of cooking. Chilling to 3ºC should be completed within 90 min and the food should be stored at 0–3ºC.

• In the cook–pasteurise–chill system, hot food is filled into a flexible container, a partial vacuum is formed to remove oxygen and the pack is heat sealed. It is then pasteurised to a minimum temperature of 80ºC for 10 min at the thermal centre, followed by immediate cooling to 3ºC. These foods have a shelf life of 2–3 weeks

7.2 Equipment

• Chilling equipment is classified by the method used to remove heat, into:

• mechanical refrigerators

• cryogenic systems.

7.2.1 Mechanical refrigerators

• Mechanical refrigerators have four basic elements: an evaporator, a compressor, a condenser and an expansion valve

• In the evaporator: the liquid refrigerant evaporates under reduced pressure, and in doing so absorbs latent heat of vaporization and cools the freezing medium. This is the most important part of the refrigerator; the remaining equipment is used to recycle the refrigerant.

• Refrigerant vapour passes from the evaporator to the compressor where the pressure is increased.

• The vapour then passes to the condenser where the high pressure is maintained and the vapour is condensed.

• The liquid passes through the expansion valve where the pressure is reduced to restart the refrigeration cycle.

The important properties of refrigerants are as follows:

• a low boiling point and high latent heat of vaporisation

• a dense vapour to reduce the size of the compressor

• low toxicity and non-flammable

• low miscibility with oil in the compressor

• low cost.

• Ammonia has excellent heat transfer properties and is not miscible with oil, but it is toxic and flammable, and causes corrosion of copper pipes.

• Carbon dioxide is nonflammable and non-toxic,2 making it safer for use for example on refrigerated ships, but it requires considerably higher operating pressures compared to ammonia.

• Halogen refrigerants (chlorofluoro-carbons or CFCs) are all non-toxic and non-flammable and have good heat transfer properties and lower costs than other refrigerants.

Properties of refrigerants

• main refrigerants that are now used are Freon-22 and ammonia, with the possibility of future use of propane. However, the latter two in particular are more expensive and could cause localised hazards, thus requiring additional safety precautions and training for equipment users

7.2.2 Cryogenic chilling

• A cryogen is a refrigerant that changes phase by absorbing latent heat to cool the food. Cryogenic chillers use solid carbon dioxide, liquid carbon dioxide or liquid nitrogen. Solid carbon dioxide removes latent heat of sublimation (352 kJ kg1 at 78ºC), and liquid cryogens remove latent heat of vaporisation (358 kJ kg1 at 196ºC for liquid nitrogen; liquid carbon dioxide has a similar latent heat to the solid). The gas also absorbs sensible heat as it warms from 78ºC (CO2) or from 196ºC (liquid nitrogen) to give a total refrigerant effect of 565 kJ kg1 and 690 kJ kg1 respectively.

• The main limitation of carbon dioxide, and to a lesser extent nitrogen, is its ability to cause asphyxia. There is therefore a maximum safe limit for operators of 0.5% CO2 by volume and excess carbon dioxide is removed from the processing area by an exhaust system to ensure operator safety, which incurs additional setup costs. Other hazards associated with liquefied gases include cold burns, frostbite and hypothermia after exposure to intense cold

Chill storage

• Once a product has been chilled, the temperature must be maintained by refrigerated storage. Chill stores are normally cooled by circulation of cold air produced by mechanical refrigeration units, and foods may be stored on pallets, racks, or in the case of carcass meats, hung from hooks.

Control of storage conditions

• In all stores it is important to maintain an adequate circulation of air using fans, to control the temperature, relative humidity or atmospheric composition. Foods are therefore stacked in ways that enable air to circulate freely around all sides. This is particularly important for respiring foods, to remove heat generated by respiration or for foods, such as cheese, in which flavour development takes place during storage. Adequate air circulation is also important when high storage humidities are used for fresh fruits and vegetables

Temperature monitoring

• Temperature monitoring is an integral part of quality management and product safety management throughout the production and distribution chain. Improvements to microelectronics over the last ten years has enabled the development of monitoring devices that can both store large amounts of data and integrate this into computerized management systems

Effect on foods

• The most significant effect of chilling on the sensory characteristics of processed foods is hardening due to solidification of fats and oils.

• enzymic browning, lipolysis, colour and flavour deterioration in some products and retrogradation of starch to cause staling of baked products

• Lipid oxidation is one of the main causes of quality loss in cook–chilled products, and cooked meats in particular rapidly develop an oxidised flavour termed ‘warmed-over flavour’ (WOF),

• Physico-chemical changes including migration of oils from mayonnaise to cabbage in chilled coleslaw, syneresis in sauces and gravies due to changes in starch thickeners, evaporation of moisture from unpackaged chilled meats and cheeses, more rapid staling of sandwich bread at reduced temperatures and moisture migration from sandwich fillings may each result in quality deterioration

Loss of vitamins during chillingLoss of vitamins during chilling

Chapter 9. Chapter 9. Freezing

• Freezing is the unit operation in which the temperature of a food is reduced below its freezing point and a proportion of the water undergoes a change in state to form ice crystals. The immobilisation of water to ice and the resulting concentration of dissolved solutes in unfrozen water lower the water activity (aw) of the food

• Preservation is achieved by a combination of low temperatures, reduced water activity and, in some foods, pre-treatment by blanching.

The major groups of commercially frozen foods are as follows:

• fruits (strawberries, oranges, raspberries) either whole or pureed, or as juice concentrates

• vegetables (peas, green beans, sweet corn, spinach, and potatoes)

• fish fillets and sea foods (cod, plaice, shrimps and crab meat) including fish fingers, fish cakes or prepared dishes with an accompanying sauce

• meats (beef, lamb, poultry) as carcasses, boxed joints or cubes, and meat products (sausages, beefburgers, reformed steaks)

• baked goods (bread, cakes, fruit and meat pies)• prepared foods (pizzas, desserts, ice cream, complete

meals and cook–freeze dishes).

8.1 Theory

• During freezing, sensible heat is first removed to lower the temperature of a food to the freezing point. In fresh foods, heat produced by respiration is also removed. This is termed the heat load, and is important in determining the correct size of freezing equipment for a particular production rate.

• A substantial amount of energy is therefore needed to remove latent heat, form ice crystals and hence to freeze foods.

• The latent heat of other components of the food (for example fats) must also be removed before they can solidify but in most foods these other components are present in smaller amounts and removal of a relatively small amount of heat is needed for crystallisation to take place

Time–temperature data during freezing.

• AS The food is cooled to below its freezing point f which, with the exception of pure water, is always below 0ºC . At point S the water remains liquid, although the temperature is below the freezing point. This phenomenon is known as supercooling and may be as much as 10ºC below the freezing point.

• SB The temperature rises rapidly to the freezing point as ice crystals begin to form and latent heat of crystallisation is released.

• BC Heat is removed from the food at the same rate as before, but it is latent heat being removed as ice forms and the temperature therefore remains almost constant. The freezing point is gradually depressed by the increase in solute concentration in the unfrozen liquor, and the temperature therefore falls slightly. It is during this stage that the major part of the ice is formed .

• CD One of the solutes becomes supersaturated and crystallizes out. The latent heat of crystallization is released

• DE Crystallisation of water and solutes continues. The total time tf taken (the freezing plateau) is determined by the rate at which heat is removed.

• EF The temperature of the ice–water mixture falls to the temperature of the freezer. A proportion of the water remains unfrozen at the temperatures used in commercial freezing; the amount depends on the type and composition of the food and the temperature of storage. For example at a storage temperature of -20ºC the percentage of water frozen is 88% in lamb, 91% in fish and 93% in egg albumin.

Equipment

Freezers are broadly categorized into:

• mechanical refrigerators, which evaporate and compress a refrigerant in a continuous cycle and use cooled air, cooled liquid or cooled surfaces to remove heat from foods

• cryogenic freezers, which use solid or liquid carbon dioxide, liquid nitrogen (or until

recently, liquid Freon) directly in contact with the food.

Cooled-air freezers

• chest freezers food is frozen in stationary (natural-circulation) air at between -20ºC and -30ºC. Chest freezers are not used for commercial freezing owing to low freezing rates (3–72 h).

• A major problem with cold stores is ice formation on floors, walls and evaporator coils, caused by moisture from the air or from unpackaged products in the store.

blast freezers:

• air is recirculated over food at between -30ºC and -40ºC at a velocity of 1.5–6.0 m s1. The high air velocity reduces the thickness of boundary films surrounding the food and thus increases the surface heat transfer coefficient

• In batch equipment, food is stacked on trays in rooms or cabinets. Continuous equipment consists of trolleys stacked with trays of food or on conveyor belts which carry the food through an insulated tunnel. The trolleys should be fully loaded to prevent air from bypassing the food through spaces between the trays

• Belt freezers (spiral freezers) have a continuous flexible mesh belt which is formed into spiral tiers and carries food up through a refrigerated chamber. In some designs each tier rests on the vertical sides of the tier beneath and the belt is therefore ‘selfstacking’. This eliminates the need for support rails and improves the capacity by up to 50% for a given stack height.

• Fluidised-bed freezers are modified blast freezers in which air at between -25ºC and -35ºC is passed at a high velocity (2–6 m s1) through a 2–13 cm bed of food, contained on a perforated tray or conveyor belt. In some designs there are two stages; after initial rapid freezing in a shallow bed to produce an ice glaze on the surface of the food, freezing is completed on a second belt in beds 10–15 cm deep.

Cooled-liquid freezers

• In immersion freezers, packaged food is passed through a bath of refrigerated propylene glycol, brine, glycerol or calcium chloride solution on a submerged mesh conveyor

Cooled-surface freezers

• Plate freezers consist of a vertical or horizontal stack of hollow plates, through which refrigerant is pumped at ----40ºC . They may be batch, semi-continuous or continuous in operation. Flat, relatively thin foods (for example filleted fish, fish fingers or beef burgers) are placed in single layers between the plates and a slight pressure is applied by moving the plates together.

Plate freezer

• Scraped-surface freezers are used for liquid or semi-solid foods (for example ice cream). They are similar in design to equipment used for evaporation and heat sterilization but are refrigerated with ammonia, brine, or other refrigerants. In ice cream manufacture, the rotor scrapes frozen food from the wall of the freezer barrel and simultaneously incorporates air. Alternatively, air can be injected into the product. The increase in volume of the product due to the air is expressed as overrun

Cryogenic freezers

• Freezers of this type are characterised by a change of state in the refrigerant (or cryogen) as heat is absorbed from the freezing food. The heat from the food therefore provides the latent heat of vaporisation or sublimation of the cryogen. The cryogen is in intimate contact with the food and rapidly removes heat from all surfaces of the food to produce high heat transfer coefficients and rapid freezing. The two most common refrigerants are liquid nitrogen and solid or liquid carbon dioxide.

Liquid-nitrogen freezer

Changes in foods

Effect of freezing

• The main effect of freezing on food quality is damage caused to cells by ice crystal growth. Freezing causes negligible changes to pigments, flavours or nutritionally important components, although these may be lost in preparation procedures or deteriorate later during frozen storage.

Effect of freezing on plant tissues: (a) slow freezing; (b) fast freezing.

The main changes to frozen foods during storage are as follows:

• Degradation of pigments. Chloroplasts and chromoplasts are broken down and chlorophyll is slowly degraded to brown pheophytin even in blanched vegetables. In fruits, changes in pH due to precipitation of salts in concentrated solutions change the colour of anthocyanins.

• Loss of vitamins. Water-soluble vitamins (for example vitamin C and pantothenic acid) are lost at sub-freezing temperatures .

• Vitamin C losses are highly temperature dependent; a 10ºC increase in temperature causes a sixfold to twentyfold increase in the rate of vitamin C degradation in vegetables and a thirtyfold to seventyfold increase in fruits. Losses of other vitamins are mainly due to drip losses, particularly in meat and fish (if the drip loss is not consumed).

• Residual enzyme activity. In vegetables which are inadequately blanched or in fruits, the most important loss of quality is due to polyphenoloxidase activity which causes browning, and lipoxygenases activity which produces off-flavours and off-odours from lipids and causes degradation of carotene. Proteolytic and lipolytic activity in meats may alter the texture and flavour over long storage periods.

• Oxidation of lipids. This reaction takes place slowly at -18ºC and causes off-odors and off-flavors.

Chapter 10: Process control

• The purpose of process control is to reduce the variability in final products so that legislative requirements and consumers’ expectations of product quality and safety are met

• It also aims to reduce wastage and production costs by improving the efficiency of processing.

• Simple control methods (for example, reading thermometers, noting liquid levels in tanks, adjusting valves to control the rate of heating or filling), have always been in place, but they have grown more sophisticated as the scale and complexity of processing has increased

• With increased mechanisation, more valves need to be opened and more motors started or stopped. The timing and sequencing of these activities has become more critical and any errors by operators has led to more serious quality loss and financial consequences. This has caused a move away from controls based on the operators’ skill and judgement to technology-based control systems.

Automatic control has been developed and applied in almost every sector of the industry. The impetus دافع for these changes has come from:

• increased competition that forces manufacturers to produce a wider variety of products and more quickly

• escalating ارتفاع labour costs and raw material costs

• increasingly stringent قاصي regulations that have resulted from increasing consumer demands for standardised, safe foods and international harmonisation ا�نسجام of legislation and standards.

• For some products, new laws require monitoring, reporting and traceability of all batches produced which has further increased the need for more sophisticated process control.

• All of these requirements have caused manufacturers to upgrade the effectiveness of their process control and management systems. Advances in microelectronics and developments in computer software technology, together with the steady reduction in the cost of computing power, have led to the development of very fast data processing.

Automatic control

• Automation means that every action that is needed to control a process at optimum efficiency is controlled by a system that operates using instructions that have been programmed into it.

The advantages of automatic process control can be summarised as:

• more consistent product quality (minor variations in processing that would cause changes to product quality are avoided)

• greater product stability and safety

• greater compliance with legal and customer specifications

• more efficient operation

• verification of correct inputs by operators (e.g. checking that operators specify the correct weight and types of ingredients)

• better use of resources (raw materials, energy, labour, etc.)

• increased production rates (e.g. through optimization of equipment utilization)

• improved safety (automatic and rapid fail-safe procedures with operator warnings in case of, for example, a valve failure or excessive temperature rise).

Other disadvantages include:

• not suitable for processes in which manual skill is necessary or economically more attractive

• higher set-up and maintenance costs

• increased risk, delays and cost if the automatic system fails

• the need for a precise understanding of the process for programming to achieve the required product quality

• reliance on accurate sensors to precisely measure process conditions.

The components of an automatic control system are as follows:

• sensors to detect process conditions and transmitters to send this information to the controller

• a controller to monitor and control a process

• actuators (for example a motor, solenoid or valve) to make changes to the process conditions

• a system of communication between a controller and actuators and transmitters

• an ‘interface’ for operators to communicate with the control system.

components of an automatic system

• Sensors

The pre-requisites for control of a process are sensors and instruments which measure specified process variables and transmit the information to a process controller.

Parameters commonly measured by sensors are classified into:

• primary measurements (for example temperature, weight, flow rate and pressure)

• comparative measurements, obtained from comparison of primary measurements (for example specific gravity)

• inferred استنتاجي measurements, where the value of an easily measured variable is assumed to be proportional to a phenomenon that is difficult to measure (for example hardness as a measure of texture)

• calculated measurements, found using qualitative and quantitative data from analytical instruments or mathematical models (for example biomass in a fermenter).

Examples of measured parameters and types of sensors used in the control of food processes

It is important to recognize that process variables that are measured and controlled are often only used indirectly as indicators of complex biochemical reactions that take place during processing. Examples include :

• the combination of time and temperature needed to

destroy micro-organisms

• temperature and pressure as measures of the changes that take place during extrusion or the time required to remove a

• specified amount of moisture by dehydration

Controllers

• The information from sensors on process and product variables is used by controllers to make changes to process conditions.

Computer-based systems

• The increasingly widespread use of microprocessor-based process controllers over the last twenty years is due to their flexibility in operation, their ability to record (or ‘log’) data for subsequent calculations and the substantial reduction in their cost

• Computers can not only be programmed to read data from sensors and send signals to process control devices, but they can also store and analyse data and be connected to printers, communications devices, other computers and controllers throughout a plant. They can also be easily reprogrammed by operators to accommodate new products or process changes.